How do I know if må ikke means mustnt or dont need to?
Context. It would be weird if you weren't allowed to support someone. You could also rephrase it: "Du kan ikke støtte meg i dette valget"(can't) vs "Du trenger ikke å støtte meg i dette valget"(don't need).
Why "Du behøver ikke støtte meg i dette valget." is not correct? I thought it means "You don't need to..."
It's correct, and among the accepted answers. Could be a glitch or a typo.